With the recent rapid spread of portable electronic devices such as mobile phones, smartphones, electronic book readers (e-book), and portable game machines, secondary batteries for drive power sources have been increasingly required to have smaller size and higher capacity. Nonaqueous secondary batteries typified by lithium-ion secondary batteries, which have advantages such as high energy density and high capacity, have been widely used as secondary batteries for portable electronic devices.
A lithium-ion secondary battery, which is one of nonaqueous secondary batteries and widely used because of its high energy density, includes a positive electrode containing an active material such as lithium cobalt oxide (LiCoO2) or lithium iron phosphate (LiFePO4), a negative electrode containing an active material such as graphite capable of receiving and releasing lithium ions, a nonaqueous electrolytic solution in which an electrolyte formed of a lithium salt such as LiBF4 or LiPF6 is dissolved in an organic solvent such as ethylene carbonate or diethyl carbonate, and the like. The lithium-ion secondary battery is charged and discharged in such a way that lithium ions in the secondary battery move between the positive electrode and the negative electrode through the nonaqueous electrolytic solution and inserted into or extracted from the active materials of the positive electrode and the negative electrode.
A binder is mixed into the positive electrode or the negative electrode in order that active materials can be bound to each other and an active material layer and a current collector can be bound. Since the binder is generally an organic high molecular compound such as polyvinylidene fluoride (PVdF) which has an insulating property, the electrical conductivity thereof is extremely low. Thus, as the proportion of the binder to the active material is increased, the proportion of the active material in the electrode is relatively decreased, resulting in lower discharge capacity of the secondary battery.
Hence, by mixture of a conductive additive such as acetylene black (AB) or graphite particles, the electrical conductivity between active materials or between an active material layer and a current collector is improved. Thus, an active material layer with high electrical conductivity can be provided (see Patent Document 1).
An electrode containing graphene as a conductive additive has been developed. Patent Document 2 discloses an electrode manufacturing method including a step of mixing graphene oxide (also referred to as GO (short for Graphene Oxide)), an active material, and a binder and then reducing GO. By this manufacturing method, an active material layer having high electrical conductivity only with a small amount of the conductive additive can be provided.